Edit STL file and perform a very simple cut
Well, as you seem to have discovered, there is no shortage of possible ways to go about this.

Without going into too much detail, I would probably use a boolean modifier in this case. To do this:


Import your STL file (File > Import > STL)
Create an object roughly in the shape of the hole you'd like to create in the STL.
To do this: 


Add a cube (&#8679; ShiftA> Mesh > Cube)
Scale it on the X axis to make it thinner (S to scale, X to restrict the scaling solely to the X axis,  LMB or &#9166; Enter to confirm and exit the scale operator)
Grab (G), rotate (R) and scale (S) the cube into position such that it intersects the buckle object where the hole should go.



Add a boolean modifier.
To do this:


Select the buckle object (click on it with  RMB) and add a  boolean modifier in Properties > Modifiers
Set the object added in step 2 as the "Target" object in the boolean modifier
Set the boolean operation to "Difference"
(Optional) Set the cube's Maximum Draw Type to Wire in Properties > Object > Display, so you can see the result of the boolean operation better



Adjust the cube further if needed
Export the result. Either Apply the boolean modifier and delete the cube object before doing this, or enable Selection Only in the export options and ensure that the cube is not selected.